While organic electroluminescent (EL) devices have been known for over two decades, their performance is generally limited due to several adverse effects. In its simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are commonly referred to as organic light-emitting diodes (OLEDs). Today, the most efficient emitters are phosphorescent ones, where the radiative transition is from a triplet state to the ground state. Usually, Iridium or Platinum complexes are used in state-of-the art OLEDs. Normally, between 5% and 15% per weight of the phosphorescent emitter are doped into a material. The use of phosphorescent emitters places a strict requirement on the triplet level of the matrix material.
If the triplet level of the matrix is lower than the triplet level of the emitter, energy is transferred from emitter to matrix, quenching the emission of light. Only if the triplet level of the matrix is higher than the triplet level of the emitter, the energy transfer is inhibited and the resulting OLED exhibits is high quantum efficiency. Quantum efficiency is the ratio of injected charge carrier pairs into the OLED and extracted photons.
For red, yellow and yellow-green emitters this requirement is not difficult to meet, since the triplet level of most matrix materials corresponds to an emission wavelength of 520-550 nm. Only for emitters with wavelengths shorter than 520 nm and especially shorter than 470 nm, there are only few suitable matrix materials.
Since many of the phosphorescent metal complexes conduct electrons well, matrix materials conducting holes are used.
Consequently, there is the constant need for hole-conducting materials with high triplet levels.